Today's usual heating and air-conditioning systems are mostly so-called high-pressure induction systems for a whole house. There is a central air processing system which does not, however, process the total air quantities of fresh-air and stale air but only a part thereof; namely, the fresh-air taken in from outside. In this central air processing system, the fresh-air is filtered and pre-heated, or as necessary, pre-cooled and humidified. A fresh air ventilator conducts the intaken fresh-air over an insulated channel system to so-called air re-treatment units in which the fresh-air is mixed with the stale air, that is with the air already in the system, and is re-heated or re-cooled as necessary. The amount of fresh-air, hereafter also called primary air, is so selected that it is approximately equal to the air which should be renewed which is in the room adjoining the system. The air to be renewed is extracted by a ventilator and conveyed into the open. The recovery of the heat contained in the expelled air can be carried out by a heat exchange aggregate which serves to preheat the fresh air.
There are many disadvantages of systems with a central air processing arrangement of this type which require a relatively large amount of planning. Exact coordination of the work to be carried out by heating/air conditioning experts, sanitary installation workers and electricians is necessary. A relatively large amount of space is required by the air processing system and air channels and, in view of the air channels, most often being covered by higher rooms. There is a greater transmission of noise from room to room over the air channels. Furthermore, the entire system must operate fully even if only certain rooms in the building should be air conditioned.